I went through this last year as well. Got pregnant in September, only 13 months after opening the daycare. I waited 12 weeks to tell my parents, which I believe is ok to do. I met with all parents individually using the pretext of wanting to discuss care, progress, routine and generally discuss anything they wish (it was a great meeting with everyone)
As discussed above make sure to have a very concrete plan to present as they will be concerned, altough very happy for you.
My plan was that my spouse take 9 months parental leave, and I took two weeks summer vacation after birth. My spouse is returning to work next week... YIKES! Everything worked out very well.
Make sure to consider (as someone above suggested) appointments through pregnancy, if you plan on stopping when giving birth, on due date or before (all my parents were eagerly waiting as they were notified my two weeks vacation started at birth.... thank god for that as she was 7 days late). If you chose to continue providing care afterwards you may want to revise your schedule and have an honest and open discussion with the parents about the changes to come (even at the last parts of the pregnancy).
The last month of daycare was very difficult as my hips hurt so much, lifting kids for changing, toileting and putting them in their playpens was so difficult. I had to really on spouse and family for help on all other things as I was completely wiped after the day.
Good luck and congrats.